The antenna emitting power of a cellular radio telephone such as a mobile telephone can be increased in a known way by arranging a radio frequency power amplifier, a so called booster or booster amplifier, between the cellular radio telephone and the antenna. In practice it is also necessary to arrange for amplification of the signal received in the booster. Due to cost constraints the booster construction is kept as simple as possible, so that it comprises only the necessary power amplifiers.
It is also possible to compensate for the position sensitivity in a weak field by arranging a diversity reception in a known way, e.g. using two antennas. When the signal received by one antenna "fades", the stronger signal received by the other antenna can be connected to the receiver.
When it is desired to combine diversity reception with the booster solution, one straightforward solution is to duplicate the amplifier circuits, i.e. to arrange amplifiers for both receive antennas. (On the transmit side still only one power amplifier and one of the antennas is used to transmit the radio signal.) Then the selection of the better signal is made in the cellular telephone as opposed to the booster. The cellular radio telephone is provided with sufficiently versatile and intelligent circuits to perform the task.
Then we have the problem of a duplicated connection between the telephone and the booster.
An alternative diversity booster would comprise complete transmit/receive circuits in the booster, but because of its cost this solution is not feasible.
It is known e.g. from U.S. Pat. No. 4,513,412 to provide a selection of antenna at a portable or at a base station, whereby both stations will send a preamble over both antennas, that is two preambles will follow each other in successive time slots. The qualities of the preambles will determine which antenna is selected.
However, with this known arrangement, and in digital cellular radio telephone systems generally, problems are caused by the synchronization or timing required by the receive and transmit functions, because the signals are sent as short bursts, which have to be identified also during reception.
Signals are generally transmitted between base stations and mobile stations in cellular radio telephone systems using so-called multiple-access systems. One such system is known as Frequency-division Multiple Access (FDMA) in which the available radio frequency bandwidth is divided into channels of a predetermined bandwidth and the individual channels are assigned on demand, on a first-come first-served basis. As each call is made a channel is provided for the entire duration of the call and information may be transmitted continuously. In an alternative system known as Time-division Multiple Access (TDMA) the frequency channels are further divided into a predetermined number of sequential time slots only one of which is provided for an individual user. Therefore, information must be transmitted in sequential bursts at pre-determined time intervals arranged to coincide with the user's time slots. TDMA provides the advantage of additional capacity but requires additional electronics within the base station and mobile stations to enable temporal control of the transmission of information.